Prepared for the Tree Society of Zimbabwe 1995

EUCALYPTS IN HARARE

Lyn Mullin

Prepared for the Tree Society of Zimbabwe 1995

Revised and updated 26 June 1998

INTRODUCTION

The [British South Africa] Company...recognising

the desirability of tree-planting, established a

nursery on the eastern bank of the Chirapagura

stream (at present-day Newlands) and, in due

course, the [Sanitary] Board was provided with

gum-trees which were planted on the edge of the

vlei and along the drain, but the cows, which

roamed at will, ate them.

GH TANSER - A SCANTLING OF TIME

The events recorded here seem to have occurred early in 1892, and give us the first known account of eucalypt planting in Harare - or anywhere else in Zimbabwe. Since then eucalypts have become so much a part of the Zimbabwean landscape that Mashonaland, in particular, would now seem strange without them. From practically every vantage point on the highveld eucalypts can be seen as plantations, farm woodlots, or solitary trees standing high above the indigenous vegetation. To the Shona people they have become mapuranga or magamutirii, and to the English-speaking people they are gums, blue gums (often said as one word), or bluegumtrees.

But back to Harare. Mid-1896

The [Sanitary] Board decided that tree planting

was an essential matter...The contractor who had

undertaken to plant 3,000 trees per month had

failed to do so, and now a full time employee of

the Board was appointed. A large plantation of

blue gums was made at the south-east corner of

the town, and a new tree planting scheme started

in the streets.

GH TANSER - A SCANTLING OF TIME

June 1897

[Earl] Grey wrote to [Sir Alfred] Milner, "We are

going to celebrate the Jubilee with a fortnight's

orgy...So I have got no function for June 22nd.

I have therefore started a tree planting day in

honour of the Queen. Every stalwart who wishes to

show his loyality to the Queen and plants his

tree on the Queen's day must dig his own hole.

No black or paid labour allowed, as the labour

required for the digging of the holes is to be

regarded as a voluntary act of individual homage

to the Queen...

Grey's appeal to the residents to show their

loyalty by planting a tree in a hole which each

of them had dug fell on deaf ears; but a short

time before Jubilee Day, he, discarding his coat

and waistcoat and watched by a number of Natives

who were greatly interested in this strange

proceeding, set to work to dig a hole in the

Hospital grounds. On Jubilee Day no one shared

his labours: Grey planted his gum-tree alone.

GH TANSER - A SCANTLING OF TIME

February 1901

As soon as the Commonage had passed into the hands

of the [Town] Council, a number of plots...bordered

by the Marimba...had been surveyed. There was keen

demand by would-be leaseholders who put up grass

shelters for their cattle, planted gum trees for

shade, and sank wells...

GH TANSER - A SEQUENCE OF TIME

And so the planting of eucalypts got under way in what was to become Harare. We don't know how many different species were planted in those early days - to the settlers they would all simply have been gum trees - but we do know that among the earliest eucalypts in the present-day city were Eucalyptus citriodora (lemon-scented gum), E. grandis (flooded gum),

E. longifolia (woolybutt), E. tereticornis (forest red gum),

E. camaldulensis (river red gum), and E. paniculata (grey ironbark). Other species might have been tried without success, such as E. viminalis (manna gum), which was planted in the village of Melsetter (now Chimanimani) in 1897, and still survives there. Over the years about 200 species of eucalypts have been planted experimentally, commercially, or ornamentally in Zimbabwe, but the purpose of these notes is to describe the species that occur within Harare, hopefully to enable interested members of the Tree Society to identify them and to get to know them. But if we want to know the eucalypts we commonly see, we really need to understand something of their background and taxonomy. This should provide an apprec- iation of the nature of the eucalypts and, perhaps, help to dispel some of the myths surrounding their behaviour as exotics.

HISTORY OF EUCALYPTUS

The story of the eucalypts in modern times began in the year 1642 AD, when they were noted by Abel Janszoon Tasman (1603-1659), a Dutch navigator, during his 1642-43 voyage of dis- covery of Tasmania, New Zealand, and the Tonga and Fiji Islands. In 1688 an English freebooter, William Dampier, landed on the northwest coast of Australia, and noted the occurrence of trees that exuded a gummy substance, but he confused them with the dragon tree, Dracaena draco, which produced the resin of commerce known as "dragon's blood". Then, in 1770, Captain James Cook made his first voyage to the Pacific Ocean. Attached to Cook's staff were botanist Joseph Banks and his assistant Daniel Carl Solander, who collected the first eucalypt known to science, E. gummifera (red blood- wood), from the shores of Botany Bay on the east coast of Australia, where the city of Sydney now stands. Later, they collected E. platyphylla (white gum) in north Queensland, but neither species was named at the time. On Cook's third expedition in 1777 David Nelson collected E. obliqua (messmate stringybark) on Bruny Island, off southern Tasmania, and it was this species that became the first eucalypt to be named.

Those species were probably not the first eucalypts known to western civilization, for the Portuguese, who had established a settlement on the island of Timor in 1519, would have been familiar with E. alba (white gum) growing on the coast, and

E. urophylla (Timor mountain gum) in the mountains. From those early times to the present day the eucalypts have become widely known and planted in practically every country that has a subtropical or mediterranean-type climate, and even in some regions that experience severe winters, and they have been the subjects of intensive study and massive volumes of scientific literature. New species are still being discovered and described, and today the number of validly named species stands at about 725.

The generic name Eucalyptus was coined from two Greek words, eu (well) and kalyptos (covered), in allusion to the operc- ulum, or cap, covering the flower bud. When the flower bud is fully developed this operculum is shed to expose the stigma, and the stamens with their anthers.

NATURAL OCCURRENCE OF EUCALYPTUS

The eucalypts have been described as Australia's gift to man- kind, but, while it is true that the overwhelming majority of the 725 or so species are Australian endemics, perhaps 9-10 extend northwards into the neighbouring islands, and 5-6 specific and infraspecific taxa do not occur in Australia at all. Among the latter is E. deglupta (kamerere), which occurs from New Guinea northwards to Mindanao in the Philippines, the only eucalypt to cross the equator into the northern hemisphere.

Essentially, though, the eucalypts are Australian. They dominate the Australian continent to an extent that few other trees have done in any other country, and they give the Australian landscape its distinctive appearance. But there are parts of the continent where eucalypts are notably absent: they are excluded from the tropical and subtropical rain-forests of Queensland and New South Wales, and from the temperate rainforests of Victoria and Tasmania; they cannot survive the intense cold and the strong winds of the highest summits of southeastern Australia; and they do not occur commonly in the extensive arid zones that cover nearly 50% of the Australian mainland, although a few species may be locally abundant along watercourses in those regions.

Eucalypts do not extend westwards to the Asian side of the Wallace Line, nor eastwards to New Caledonia or New Zealand. Within Australia the most southerly occurrence of the eucalypts is at latitude 43 degrees South in Tasmania, and although most of the Tasmanian species also occur on the mainland, there are about a dozen that are confined to the island, probably by climatic factors resulting from the southerly latitudes and the fairly high altitudes.

Most eucalypts grow in localities where there is a marked shortage of soil moisture for much of the year, and they are well adapted to drought. At the more arid side of their environment the species that do occur there have adopted specialized life-forms, in particular the form known as mallee, in which the plant is no longer a single-stemmed tree, but one with numerous slender stems arising from a large underground rootstock that is well protected from fire. Scattered trees of larger size may be found in the mallee areas, and the mallee gives way to better-quality eucalypt woodland as rainfall improves. The woodlands contain a wide variety of species, and they, in turn, give way to open forests of taller trees, but fewer species, as conditions improve further still. Finally, the best soils in the wetter regions of southeastern ansd southwestern Australia carry magnificent closed forests consisting of a few, sometimes only one, eucalypt species, with tree heights reaching 60 metres or more.

MATURE SIZE OF EUCALYPTS

The eucalypts show great variation in mature size, from one-metre shrubs like E. cunninghamii (cliff mallee ash) to the giant E. regnans (mountain ash) towering more than 100 metres, and the tallest hardwood species in the world. The present-day champion mountain ash is a specimen in Tasmania with a height of 99 metres, but there is a record of a tree felled in Victoria in 1872 that measured 132 metres to the point where the top had broken off in the fall. That measurement was never properly authenticated and is very rarely quoted, but there is a reliable record, also from Victoria, of a tree felled in 1918 that measured 114 metres. Other measurements of mountain ash of 107, 104, 99, and 98 metres have been recorded. From Western Australia there has been a fairly reliable record for a specimen of E. diversicolor (karri) that measured 104 metres when felled in 1901, while the tallest known standing karri today has a height of 88 metres.

The tallest eucalypt in Zimbabwe is a specimen of E. grandis (flooded gum) planted in 1905 on the farm Inodzi, near Penhalonga in the Eastern Highlands. In 1984 this tree was measured with instruments and found to have a height of 73 metres. Another specimen of E. grandis, at Tilbury Estate in the Chimanimani District, was found to have a height of 74 metres in 1988, but that measurement has not been verified.

EVOLUTION OF EUCALYPTUS

Fossil evidence indicates that a eucalypt type made its appearance in Australia about 25 million years ago. That was long after the break-up of Gondwanaland (135 million years ago) and the final separation of the Australian landmass from Antarctica (55 million years ago), which would explain why the eucalypts are almost exclusively Australian. The northwards migration of the eucalypts into New Guinea, the Lesser Sunda Islands, and the Philippines became possible after the Austr-alian plate collided with the Asian plate about 15 million years ago.

At the break-up of Gondwanaland Australia was largely covered by cool-temperate rainforest, but as the continent drifted northwards and eastwards to its present position, there were changes in its climate, and massive erosion of its surface, which left Australia as one of the driest and flattest land surfaces in the world. The impact on the Australian vegetation was immense, and it would seem likely that the great profusion of Eucalyptus species evolved as a consequence of those climatic changes following rapidly on one another. The eucalypts evolved because Australia had become dry. And it is against this background of the remorseless deterioration of the Australian environment that we can begin to appreciate what outstanding evolutionary capacity the progenitors of the eucalypts must have had in order to keep pace with the environmental changes of their habitat.

There are many links between the flora of Australia, South America, and southern Africa that show the Gondwanaland connection, but eucalypts, or anything like them, are entirely absent from South America and southern Africa, although the botanical family to which they belong, Myrtaceae, is present in both; for example, our well-known waterberries, Syzygium spp, belong to that family.

Three powerful factors have been at work in shaping the eucalypts. Firstly, they evolved as a response to declining soil fertility associated with the erosion, weathering, and laterization of the Australian soils; secondly, they were subjected to a rapidly drying climate; and, finally, they had to cope with fire.

Up to this point we have been looking at Eucalyptus as a single genus, but there is evidence to show that the eucalypts may have evolved from more than one progenitor or prototype. Scientific opinion on the evolution and development of Eucalyptus has swung from the early single-genus approach to a concept of up to nine genetically independent subgenera (or even genera), then to a belief that only two genera were involved, which led to the revolutionary step by some botanists to excise, as a separate genus Corymbia, two of the nine subgenera that had earlier been recognized. Currently, a new phylogenetic approach to the eucalypts recognizes 13 sub- genera, including the previously independent genus Angophora. With the passage of time and further study the complete picture will undoubtedly emerge. If it is finally concluded that Eucalyptus comprises more than one genus, the very close similarities between the species of the different genera will have to be acknowledged as the result of evolutionary converg-ence: this is seen particularly in the development of the operculum in all species of all groups, of bark shedding of some species of each group, and in the absence in some species of some groups of the woody swellings (lignotubers) that occur at the base of the tree, and provide a reserve of buds and food to assist survival under adverse conditions.